Concatenation is what i think your trying to do, maybe don't bother :), making partitions is pointless a lot of the time imo.
If it were me, put OS and programs on the fastest drive, make the whole thing C:, and whatever 7 also wants to put there...
Put your data (downloads) on the 1.5tb (D:), as well maybe backup your C: on a folder on the D:, and backup things you can't easily replace like personal pictures/documents on the USB from time to time, keep that away in a safe location.
just keep that shit backed up man, i use a program called Unison, easy to use after initial setup...
http://www.cis.upenn.edu/~bcpierce/unison/
an u can make multiple drives not in raid show up as one fuck